This invention relates to a method for cold-bending, to a desired curvature, resin-coated steel pipes having a rib or ribs, formed of the same material as the coating, which rib or ribs are formed on the outer periphery of such pipes, especially such pipes having a rib or ribs consisting of a pair of parallel projections that form between such projections a hold groove for holding a wire net or a panel. More particularly, this invention relates to curved resin-coated ribbed pipes bent by this method.
Lately, there has been an increasing demand for hand-pushed trolleys having side walls with wire nets as shown in FIG. 9, and for transparent dome-shaped covers for safety, water tightness or for dust tightness. To meet such demand, resin-coated steel pipes having a rib or ribs for holding a wire net or a transparent panel have been developed. Samples of cross-sections of resin-coated steel pipes are shown in FIG. 8 A-D. The resin-coated steel pipe 1A shown in FIG. 8A is a circular thin steel pipe 10 having an outer diameter of 25.7 mm and a thickness of 0.7 mm, with a uniform coating of about 1 mm of thermoplastic resin, such as AAS or ABS resin, and a rib 12 consisting of a parallel pair of radially protruding equal projections 12a, 12a formed from the same resin as the coating 11. The projections form a hold groove 13 with a width (3 mm) and a depth (11 mm) capable of holding a panel or other such item or device.
The resin-coated steel pipe 1B shown in FIG. 8B has two radially protruding ribs 12 provided on the outer surface of a circular steel pipe 10, one each at diametrically opposite positions, each such rib consisting of two congruent projections 12a, 12a formed from the same resin as the coating 11 and each forming a hold groove 13 having a depth and a width capable of holding a panel or other such item or device.
The resin-coated steel pipe 1C shown in FIG. 8C has two ribs 12 provided at a right angle on the outer surface of a circular steel pipe 10, each such rib consisting of two radially protruding congruent projections 12a, 12a formed from the same resin as the coating 11 and each forming a hold groove 13 having a depth and a width capable of holding a panel or other such item or device.
The resin-coated steel pipe 1D shown in FIG. 8D has three ribs 12 each provided at an angular spacing of a right angle on the outer surface of a circular steel pipe 10, each such rib consisting of two radially protruding congruent projections 12a, 12a formed from the same resin as the coating 11 and each forming a hold groove 13 having a depth and a width capable of holding a panel or other such item or device.
FIG. 9 and 10 show samples of resin-coated ribbed steel pipes of the kind shown in FIGS. 8A-8D bent to a desired curvature. The arched frames 1A on both sides of the hand-pushed trolley shown in FIG. 9 are the same kind of resin-coated ribbed steel pipes 1A shown in FIG. 8A bent in such a manner that the rib 12 protrudes inwardly toward the center of curvature. The arched frames 1A at both ends of the safety cover shown in FIG. 10 are also the same kind of resin-coated ribbed steel pipes 1A shown in FIG. 8A bent, however, in such a manner that the rib pair 12 protrudes laterally at a right angle to the plane of bending, while the arched center frame 1B is the same kind of resin-coated ribbed steel pipe 1B shown in FIG. 8B bent with the two ribs 12 protruding laterally.
It has been possible to bend ordinary steel pipes or resin-coated steel pipes using conventional pipe-bending machines. However, hitherto no technology or machines have been developed to bend to a desired curvature resin-coated pipes having a rib or ribs as described above.
The simple application of a conventional pipe-bending machine is not sufficient to cold-bend a resin-coated ribbed steel pipe to a desired curvature, especially to bend it in such a manner that the ribs protrude inwardly toward the center of curvature (see the arched frames in FIG. 9), because the ribs would be crushed and corrugated by compression due to the bending, thereby deforming the hold groove and making it very difficult to insert the edge of a panel or a wire net for holding.
Moreover even assuming that the resin-coated ribbed pipe is to be bent in such a manner that the ribs protrude laterally or perpendicularly to the plane of bending (see the arched center frame in FIG. 10), the projection outside the neutral plane of bending would still yield to the tension due to bending and incline inwardly, while the inner projection would incline outwardly, also toward the neutral plane, yielding to the compression due to bending, thereby narrowing or completely blocking the hold groove. This problem would also preclude the use of a conventional pipe-bending machine.
Further, it is almost impossible to hot-bend resin-coated ribbed steel pipes to a desired curvature because the resin-coating and ribs would melt long before the temperature necessary to easily bend steel pipes was attained.
It is the primary object of the present invention, therefore, to provide a method to mechanically cold-bend with precision resin-coated ribbed steel pipes and thus to produce such bent, resin-coated ribbed steel pipes.